1. Field of the Invention
The present invention relates to a multiplex communication system and more particularly to a method for controlling data relay among networks in the multiplex communication system when an event for data communication occurs.
2. Related Art
In recent years, sophistication of communication networks advances in various fields as computer technology advances. In the field of motor vehicles, the volume of information exchanged among control sections (ECUs) which control respective electrical components installed on a vehicle increases rapidly. Therefore a multiplex communication system is employed for communication among the control sections in the vehicle so that the number of wire harnesses required for the communication is reduced.
In the multiplex communication system, nodes such as ECUs or the like are connected to a common multiplex communication line and communicate each other by exchanging data frames through the common communication line. In the vehicle, a plurality of networks are formed for efficient data communication between the control sections. All nodes are appropriately divided into the networks according to the required communication speed or the like. Communication between two nodes belonging to different networks is performed via a data relay unit.
When a trigger such as an event for data communication is generated in one of the nodes, a data frame (wake-up frame) which represents that the network to which the node belongs should be activated is sent over the communication line in the network. In response to this, each of the other nodes in the network is activated and sends a wake-up frame over the communication line. In this way, by receiving a wake-up frame, each node recognizes that the event occurs in one of the other nodes in the network, and, by sending the wake-up frame, notifies the other nodes that the node is activated. If an event does not occur for a predetermined time in the network, the network (i.e., all nodes in the network) shifts into a sleep mode for minimizing power consumption.
When an event is generated in a node, the node notifies the occurrence of the event to predetermined destination nodes by generating a notice of occurrence of the event (event frame). The data relay unit receives the event frame and relays the event frame to the destination node. However, if the destination node belongs to a network which is in the sleep mode, it cannot receive the event frame. Therefore, in the case that the event frame is sent only once, the destination node often does not receive the event frame after all. When some type of event such as door-locking or door-opening in keyless entry occurs, the event frame is sent only once.
In U.S. Pat. No. 5,343,470, event frames are periodically sent many times in response to occurrence of an event so that destination node can certainly receive the event frame.
However, the destination nodes cannot necessarily receive the event frame even if the event frames are periodically sent many times. Further, traffic congestion in data communication may occur when many events occur at the same time. Moreover, certain time elapses from occurrence of the event until the destination node recognizes the occurrence of the event, resulting in delay. This delay varies greatly depending on the state of the system at the moment of the occurrence of the event.
The data relay unit may save the event frame in a buffer until the network which includes the destination node is activated. However, in this case, a high-capacity buffer is required so that the data relay unit operates effectively, and therefore cost is high.